4.Command
Handler: a CommandHandler is method and it is excuted when command is invoked.

publicpartialclassMainWindow : Window

{

...

void
_bindings_Executed(object sender, ExecutedRoutedEventArgs e)

{

MessageBox.Show("Hello");

}

}

This is a much simple example of using command but power of command come in use when UI code/presentation logic is placed into another class called ViewModal, a MVVM pattern. having Modal and UI logic handling UI events and properties.

Invalid postback or callback argument. Event validation is enabled using <pages enableEventValidation="true"/> in configuration or <%@ Page EnableEventValidation="true" %> in a page. For security purposes, this feature verifies that arguments to postback or callback events originate from the server control that originally rendered them. If the data is valid and expected, use the ClientScriptManager.RegisterForEventValidation method in order to register the postback or callback data

If you are using repeater control, make sure you have set EnableViewState ="false"